Ian Paisley is set to take the greatest gamble of his 40-year political career by challenging David Trimble head to head in the First Minister's own constituency at the general election.

Paisley is under pressure from senior strategists in his Democratic Unionist Party to stand against Trimble in Upper Bann.

The electoral battle will be a struggle for the future of unionism: Trimble's new power-sharing secular brand, or Paisley's old style emphasis on loyalty to faith and crown and No Surrender.

DUP sources told The Observer their leader would be 'parachuted in' from his own North Antrim constituency to take on Trimble in his own backyard, which includes the loyalist citadel of Portadown and Drumcree.

It is understood Paisley's son, Ian junior may stand in North Antrim, where the DUP leader polled more than 21,000 votes at the last general election.

One source in the party said: 'The big man has still to make his mind up fully but that is the plan.'

Trimble said yesterday that he was aware of the scheme. He suspected the DUP wanted a heavyweight to stand against him after it picked a relatively unknown candidate, David Simpson, for the Upper Bann contest.

The First Minister told The Observer his Ulster Unionist Party had 'thought from the start that David Simpson was just a discard. Ever since he was selected we have thought he was just there to fill a space. The big question was, had Ian Paisley Senior the bottle for it.

'I am sure I will win. I am absolutely relaxed about this. If anything my big concern is getting people out to vote. But the presence of Ian Paisley will certainly bring people out, and that will benefit me. It will do me a favour.'

Paisley's intervention in Upper Bann will be a repeat of history. In April 1970 Paisley won the Bannside seat at a Westminster by-election. The seat had previously been held by the last reformist Ulster Unionist leader prior to Trimble, Captain Terence O'Neil. One of Paisley's campaign slogans was, 'Kick O'Neil in the Bannside.'

A Paisley victory in Upper Bann would herald the beginning of the end of Trimble's tenure as UUP leader. But defeat for Paisley, even by a narrow majority, would be a severe setback to those unionists trying to kill the Good Friday Agreement.

The last time Paisley and Trimble were seen together in Upper Bann was a much more fraternal occasion for unionism. They linked arms in triumph outside an Orange Hall in Portadown after the local Orange Lodge were allowed to march down the mainly nationalist Garvaghy Road in July 1995.

Trimble said he later regretted the decision to hold Paisley's hand aloft in victory, a gesture that angered nationalists in his constituency. The UUP leader admitted he only did it in order not to be upstaged in his own constituency.

Now Trimble faces the prospect of being usurped in that same seat. Ironically, it may be nationalists who, if they vote tactically, could save him from being ousted by Paisley.

In the 1997 general election Trimble polled 20,836 votes, while the DUP candidate Mervyn Carrick took 5,482. The Alliance gained 3,617 votes, all of which may go to Trimble if the centrist party steps down in the constituency this time to stop the DUP leader winning.

The outgoing Ulster Unionist MP for Fermanagh and South Tyrone, Ken Maginnis, launched a scathing attack on the DUP yesterday over its plans to take that seat too.

Maginnis, who is standing down after 18 years at Westminster, claimed the DUP had now reached 'crisis point', and he challenged its leadership to debate publicly its alternatives to the peace process.

The DUP deputy leader Peter Robinson had earlier hinted his party was willing to withdraw its candidate from the constituency, the Stormont Minister Maurice Morrow, if it would maximise the anti-agreement vote.

Amid speculation that local man Jim Dixon was preparing to stand in the area, Robinson said he would be prepared to discuss with him how best to approach the election.

'The important thing is we should not divide the anti-agreement unionist vote in a constituency that could be lost to a pro-agreement candidate,' he told Radio Ulster.